```ts
const MODULUS = 1000000007;

/** The nth Fibonacci number, modulo 1000000007. */
export function fibMod(n: number): number {
  if (n < 0 || !Number.isInteger(n)) {
    throw new Error("Input must be a non-negative integer.");
  }

  if (n === 0) return 0;
  if (n === 1) return 1;

  let prev = 0, current = 1, next = 0;
  for (let i = 2; i <= n; i++) {
    next = (prev + current) % MODULUS;
    prev = current;
    current = next;
  }
  return current;
}
```